Of Great Magnitude

3/25/07 – 3/31/07
by C. Zaitz

“Star light, star bright, first star I see tonight…” so which one will it be? People often ask, “what’s the first star?” or “what’s the biggest star?” or my personal favorite, “what’s the farthest star?” These questions tend to our desire to catalogue things, rank them, sort and classify. Maybe knowing the biggest, best, and brightest may be a way to make sense of the infinite and chaotic universe.

The problem with far away stars is that they are really hard to see! At the very limit of our view lie the most distant objects we can see; the quasars, with their light so stretched from the billions of years of travel that their spectra have migrated far into the red and infrared extremes. Quasars are strange objects; compact, bright and very distant. They seem to be the cores of ancient galaxies, most certainly with giant black holes at their centers. As of now, the farthest quasar we’ve found is nearly 13 billion light years away. That’s close to the time when we think the first stars and galaxies were forming. Before that, there was no light at all. So we’ll have to be content to say quasars are the farthest “objects” we can see.

What about big? All stars are big, compared to earth. Maybe you’ve heard that the sun a small star. In the great scheme of things, perhaps it’s not as big as, say, Betelgeuse or Deneb, but it’s just right for us. However, there are vast numbers of loftier and heftier stars than our own. Most of the bright stars in the sky would dwarf our sun. They are bright mainly because they are huge. We call the brightness of a star its magnitude. Stars have two magnitudes; the one we see, called the apparent magnitude, and the one it really is, called the absolute magnitude. Imagine trying to compare sizes of sailboats in a lake. It would be easy if they were all the same distance from you, but they’re all over the lake. You could classify them by how big they look, but that’s not really fair for the ones far away- they’ll always look tiny. Unfortunately, it’s very tricky to find the absolute magnitude of stars since they are scattered all over the universe. With sailboats, you might be able to recognize the type of boat by its appearance and infer its size from that knowledge, and that’s what we do with stars. But there’s room for error- what if the same brand of boat comes in 24’ and 32’ and they look very similar? Astronomers have several methods of estimating size and distance, but it’s not an exact science. So we’ll just say that Epsilon Aurigae, in the constellation of Auriga, is 2,700 times bigger than our sun, and that’s one of the biggest we’ve ever seen.

So how does one decide what the first star is? Are we looking for the first star that you see in the evening sky? This answer can be evasive, since the first star is usually a planet. Planets can be very bright, especially Venus. Venus has been greeting the sunset lately, and as twilight fades, its brilliance in the western sky is unparalleled. But it’s not a star. However, when you’re looking for the first of anything in the night sky, look for Venus. Its apparent magnitude is very great, and when it comes to just enjoying the night sky, appearances can be everything!

Until next week, my friends, enjoy the view.